test_io_uring_3.cpp

#include <sys/uio.h>
#include <sys/eventfd.h>
#include <unistd.h>
#include <liburing.h>
#include <iostream>

void exit_perror(const char *s)
{
  perror(s);
  std::terminate();
}

void read_from_evfd(io_uring *ring, int fd)
{
  int64_t *buf = new int64_t;
  auto *iov = new iovec;
  iov->iov_base = static_cast<void *>(buf);
  iov->iov_len = sizeof(int64_t);
  auto *sqe = io_uring_get_sqe(ring);
  io_uring_prep_readv(sqe, fd, iov, 1, 0);
}

void sync_write_evfd(int fd)
{
  int64_t buf = 1;
  if (write(fd, &buf, sizeof(int64_t)) < 0)
    exit_perror("write");
}

int main(void)
{
  int evfd1 = eventfd(0, 0);
  int evfd2 = eventfd(0, 0);
  io_uring ring;
  io_uring_queue_init(1024, &ring, 0);
  for (;;)
  {
    read_from_evfd(&ring, evfd1);  // first submit lotta blocking workload on one fd
    read_from_evfd(&ring, evfd1);
    read_from_evfd(&ring, evfd1);
    read_from_evfd(&ring, evfd1);
    read_from_evfd(&ring, evfd1);
    read_from_evfd(&ring, evfd1);

    read_from_evfd(&ring, evfd2);  // then another workload that completes immediatly
    sync_write_evfd(evfd2);

    io_uring_submit_and_wait(&ring, 1);  // never returns with 5.4.x, returns on 5.8 & linux-next

    io_uring_cqe *cqe;
    if (io_uring_peek_cqe(&ring, &cqe) < 0)
      exit_perror("io_uring_peek_cqe");
    if (cqe->res < 0)
      exit_perror("cqe->res");
    io_uring_cqe_seen(&ring, cqe);

    std::cout << "#";
    std::cout.flush();
  }
  io_uring_queue_exit(&ring);
  return 0;
}